In recent years, various information transmissions have become possible along with a dramatic progress of information transmission technology. In particular, wireless information transmission is greatly superior and actively used from the viewpoint of convenience.
As means of wireless information transmission, cellular phone, PDA (personal digital assistance), wireless LAN, broadcasting waves, automotive radar, ETC on-vehicle equipment, various electronic devices, etc. can be cited.
On the other hand, as these wireless information transmissions spread, electromagnetic waves emitted from devices used for wireless information transmission intrude into building insides through openings of buildings to become electromagnetic noises. Therefore, there is a demand for an electromagnetic wave absorption board that can effectively absorb electromagnetic waves and has transparency.
Of wireless information transmissions, wireless LAN (local area network) is not required to have LAN works (such as cord wiring work) in the interior. Therefore, it greatly contributes to the cost reduction and ease of use in offices and general homes.
However, wireless LAN generates many problems, such as lowering of LAN speed due to the influence of reflective members (desk, locker, chair, etc.) in the interior, eavesdropping due to outdoor radio wave leak, radio wave interference among buildings and constructions (due to four channels in 2.45 GHz band region), and unauthorized access and impersonation from outside.
As countermeasures against such problems, countermeasures are conducted by exchange of issuer certificates between communication terminals, such as PC (personal computer), and servers, data encryption or periodic automatic change of cryptography key, issues of ID and passwords, etc.
However, there is no model compatibility for conducting exchange of issuer certificates, and it is difficult among different models. Countermeasures, such as data encryption, periodic automatic change of cryptography key, and ID and passwords, are accompanied with a risk of decipherment by a third person.
Therefore, they say it is necessary to prevent information leaks outside by disposing a transparent electromagnetic wave absorption board at an interior partition or an opening of buildings, constructions, etc.
Furthermore, in recent years, from the viewpoint of energy saving, there are many cases of using a double glazing, which is superior in heat insulation, at an opening. As an electromagnetic wave absorption board of such a double glazing structure, Patent Publication 1 discloses a λ/4 type electromagnetic wave absorption body in which an absorption member that is made to absorb incoming electromagnetic waves and a reflection member that is made to reflect incoming electromagnetic waves are disposed to be away from each other at a distance equivalent to ¼ of wavelength of an electromagnetic wave to be absorbed.
In the case of using a λ/4 type electromagnetic wave absorption body, the thickness of the absorption body is required to have a size of ¼ of the wavelength. Therefore, in case that the center frequency used in wireless LAN is 2.45 GHz, a thickness of about 31 mm is necessary. Therefore, there is a problem in which the thickness is too much to attach this absorption body to a partition or a window, wall, etc. of constructions.
In Patent Publication 2, there is also a proposal in which, in order to make the thickness of a λ/4 type electromagnetic wave absorption body thin, a conductive film formed in the stripe or grid form is disposed between an absorption member and a reflection member of this λ/4 type electromagnetic wave absorption body, thereby achieving the increase of the effective dielectric constant between the absorption member and the reflection member and the decrease of the thickness.
In the electromagnetic wave absorption body disclosed in Patent Publication 2, it is possible to decrease the board thickness by disposing a conductive film formed in the stripe or grid form between the absorption member and the reflection member of the λ/4 type electromagnetic wave absorption body. It is, however, complicated in its structure itself, and the preparation becomes cumbersome.
In Patent Publication 3, there are proposed one in which an electromagnetic absorption layer is disposed on the exterior side on which electromagnetic waves are incident and in which an electromagnetic reflection layer is disposed on the interior side, and furthermore one in which a liquid having a high dielectric constant is sealed in place of the hollow layer.
In this electromagnetic wave absorption glass disclosed in Patent Publication 3, it is necessary to have a conduction of the electromagnetic reflection layer with the frame. Furthermore, heat insulation becomes inferior by using a liquid having a high dielectric constant in place of the hollow layer.
Furthermore, in a single layer type, transparent, electromagnetic wave absorption body equipped with a dielectric layer and an impedance layer, a technique is disclosed in Patent Publication 4 in which an electromagnetic wave absorption effect is obtained by using a phase difference between electromagnetic waves reflected on the surface of the impedance layer and electromagnetic waves reflected on the surface of the dielectric layer. However, in case that absorption of unnecessary electromagnetic waves generated from the interior side is necessary, it is necessary to direct the conductive film surface to the exterior side. Therefore, there is a risk of deterioration in durability of the conductive film.    Patent Publication 1: Japanese Patent Application Publication 2001-44750    Patent Publication 2: Japanese Patent Application Publication 10-275997    Patent Publication 3: Japanese Patent Application Publication 5-37178    Patent Publication 4: Japanese Patent Application Publication 2003-8279